Angular2 - TypeError: Cannot read property 'Id' of undefined in (Typescript)

Question

I'm getting the following error:

angular2.dev.js:23925 EXCEPTION: TypeError: Cannot read property 'Id' of null in [


{{ product.Id }}
 in ProductEditComponent@0:68]

thrown with:

//Product-edit.component.ts:

import {Component } from 'angular2/core';
import { IProduct } from './product'
import { ProductService } from './product.service'
import { RouteParams } from 'angular2/router';
@Component({
  template:`<div class="wrapper wrapper-content animated fadeInRight ecommerce"> 
              {{ product.Id }}
            </div>`, 
})
export class ProductEditComponent{
    product: IProduct = null;
    errorMessage: string;
    constructor(private _routeParams: RouteParams, private _productService: ProductService){

    }

    ngOnInit(): void {
        this._productService.getProduct(this._routeParams.get('id'))
            .subscribe(
                product => this.product = product,
                error => this.errorMessage = <any>error);


    }

}

ProductService:

getProduct(id: string): Observable<IProduct> {
    return this._http.get(this._baseUrl + this._getProductUrl + '/' + id)
        .map((response: Response) => <IProduct>response.json())
        .do(data => console.log("All: " + JSON.stringify(data)))
        .catch(this.handleError);
}

Response from server:

{"Id":"34d4efcy6","ExternalId":null,"UserId":"testing","ProductProvider":null,"Title":"Flaska vin","Desc":null,"MinDeliveryTime":null,"MaxDeliveryTime":null,"FreightCost":null,"Brand":null}

What am I messing up?

Answer 1

In your component, you're initializing product to null, and then referencing product.Id in your template. The error occurs when Angular tries to draw your template initially, before your async call returns - at which point product is still null, thus the error: Cannot read property 'Id' of null.

The most immediate solution is to use the Elvis operator, which Angular provides for situations precisely like this. You'd use it by replacing {{ product.Id }} with {{ product?.Id }} in your template.

That said, you're likely to run into change detection problems with this approach, and in general you'll be much better off with an approach like:

export class ProductEditComponent{
  product: Observable<IProduct>; //product is an Observable
  errorMessage: string;
  constructor(private _routeParams: RouteParams, private _productService: ProductService){
     //product is always defined because you instantiate it in the ctor
     this._productService.getProduct(this._routeParams.get('id'));
  }

You'd then use {{(product | async).Id }} in place of {{product.Id}} in your template, leveraging AsyncPipe to let angular handle the dirtywork of subscribing and updating the UI as needed for you.